maybe u can find the difference from the english translation:宝宝:baby 婴儿：infant 宝宝 is more often used for oral chinese especially it is widely used for calling the baby.And you aslo can call your gf "baobao"but u cant call her"yin er".hope my answer help u a bit.(:

i will give you an example. when you want to talk about your baby with your friends,you will call your baby "宝宝"，but you will never say"婴儿", when you say "宝宝，it converts a message of love too. "“婴儿”simply means a baby which is under one year old. it is more like a general concept, when you want to say serious scientific finding about babies, you can say "婴儿".what's more ,in China, you can hear people call their kids "宝宝" even they are 4,5 year old.a 4-5 year old kid is definitely not a "婴儿"。

婴儿 n 宝宝,they both have the same meaning to describe a very little kids.but the difference is the relation between the kids n the speakers.宝宝 is a sweet name .it sounds likes called by the parents or relatives.婴儿 is used to describe a fact.

寶寶 include 嬰兒，嬰兒是指小于1周岁的孩子。寶寶是比儿童小的孩子。 You are the big baby):haha^>

You should say "我该用哪一个？“ "我的是哪一个?" means "Which one did I use?" or "Which one have I used?"